The proposed studies are designed to test the hypothesis that A1 adenosine receptors (A1ARs) activate functional effector systems in embryonic hearts to regulate cardiac activity. Further, it is proposed that A1AR activation during critical periods of heart formation affects cardiac structural development. Four specific aims are proposed. 1) The effector systems through which A1ARs influence fetal cardiac physiology will be defined. 2) A determination will be made whether A1AR activation influences cardiac structural development. 3) Transgenic mice will be generated by fusing the wild-type A1AR receptor with the atrial natriuretic factor promoter. 4) The A1AR promoter will be characterized and transgenic animals expressing high affinity A1ARs will be generated.